soil-plant relationships. That which settles directly on vegetation may remain ag surface contamination or may enter the plant through foliate absorption. When plants are eaten by animals, the radioactivity incorporated in the plants or deposited on their surfaces is absorbed and retained by the animal according to the specific metabolic characteristics of the individual nuclides. When plant and animal products are eaten by man, the radioelements they contain are absorbed and incorporated into his tissues, again in accordance with their individual metabolic properties. A few of the long-lived radionuclides in nuclear debris will be considered individually, since their accumulation in the soil and ecological transport to man appear to be the major concern, 4.1 Strontium-90 (a) Ecological Incorporation and Discrimination. Strontium-90 is chemically and metabolically similar to calcium. Therefore, it is incorporated into the biosphere along the same ecologic chain. It is taken into plants through the root system in relation to available soil calcium and absorbed and deposited in human bonein relation to the Sr®’/Ca ratio in the diet. It is reasonable to assume that strontium may be discriminated against with respect to calcium in passing along the ecological chain. Fof example, the Sr®°/Ca ratio of human bones may be expected to be lower than that of soil. Attempts have been made to determine the overall Sr®/Ca discrimination ratio in going from soils to human bone by determining the individual discrimination factors (DF) that occur at the various steps along the ecological cycle. Menzel? obtained a soil-to-plant discrimination factor (DF,) of 0.7 for four widely different soil types using both radioactive and stable strontium. Larsonand Bowen and Dymond”obtained comparable values. A discrimination factor (DF,) of 0.13 in going from plants-to-milk has been reported by Alexanderet al.?5 and Comar,”® and the discrimination factors (DF;) from plants-to-bone and from milk-to-bone (DF,) have been estimated at 0.25.27,28 The over-all discrimination ratio (OR pone-goil) in going from soil-to-human bone via the diet may be estimated from the various discrimination factors and the fraction of dietary calcium derived from dairy products and from other sources. For example, for the United States population the amount of dietary calcium derived from dairy products is estimated at about 80 per cent. The remainder is derived from cereals, vegetables, meats, etc. On the basis of the above generalizations, (ORpone-soil) = (0.8 x DF, x DF, x DF3) + (0.2 x DF, x DF,) = (0.8 x 0.7 x 0.13 x 0.25) + (0.2 x 0.7 x 0.25) = 0.05 and indicates that the average equilibrium concentration of Sr® in bone calcium for the United States population will be about 5 per cent of the concentration in the available soil calcium (Fig. 3). It should be emphasized that over-all discrimination ratios derived in the above manner apply only to passage along the ecological chain. The ecological discrimination ratio automatically assumes that calcium and strontium are uniformly mixed in soil to the average depth of the plant feeding zone. No allowance is made for direct foliar contamination, for dilution with a greater reservoir of available soil calcium through plowing, for the possibility that it may become less available with time through soil binding and leaching, or for differences in uptake by different plant species. (b) Sx*? Levels in Bones of the Population, Present and future average maximum sr® equilibrium levels in bones of the population can be estimated from the soil-to-bone discrimination ratio, the ratio of milk to other sources of calcium in the diet, and the present and predicted average maximum surface deposition levels given in the previous sections. Assuming an average of 20 g of available Ca per square foot of soil to a depth of 2.5 in., 1 mc of Sr* per square mile is equivalent to 1.8 yuc Sr® per gram of available soil calcium. If all of the Sr® is in available form, multiplication of the surface deposition levels by 1.8 gives the Sr® activity per gram of available soil calcium. Multiplication of the specific activity of available soil calcium by the Sr® discrimination ratio should give the average maximum specific activity of calcium laid down in the adult skeleton through exchange and bone remodeling during the period of environmental contamination and the average maximum sr® concentration in a skeleton at equilibrium with the integrated surface deposition levels. 291 iy :